1 Introduction

People contact me nearly every day to help make their rooms sound better, but many of them aren’t really sure if their room needs help or not. Many of them have been told they need acoustic treatment, but they are hesitant because they aren’t sure if they really do need it. Furthermore, even those who know their rooms need help often want to know how they can ‘fix’ their room while spending the smallest amount of money possible – completely understandable!

Adding to this problem of understanding what acoustic treatment can do for an audiophile or home-theater enthusiast is the fact that it’s hard to describe the sound of acoustic treatment with words; the usual issues with audiophile buzzwords (ie, ‘warm’, ‘punchy,’ ‘strident,’ etc.) are even less meaningful when it comes to the sound of a room.

This article will attempt to clear up some of these confusing aspects of room acoustics and treatment, allowing you to lay the foundation to develop an intelligent acoustic treatment strategy to maximize the sound potential of your listening room.

2 The Two Most Basic Truths About Room Acoustics

Many people I speak with have the impression that room treatment is like a toggle switch: ‘off’ is the untreated room, and ‘on’ is a perfect-sounding room. This leads to the impression described above, where people want to ‘turn the switch on’ and achieve a perfect-sounding room while spending as little as possible. Unfortunately, it doesn’t quite work this way.

This brings us to the two most basic truths about room treatment: First, no matter how well treated, there is no such thing as a perfect-sounding room. Second, no matter how good a room might sound on its own, every room can be improved with intelligently-applied acoustic treatment.

In nearly all cases, it comes down to how good you want vs. how much you can spend and how many panels you can live with in the room. The latter concern applies mostly when the listening room is also a living room, for instance, and there may be an aesthetic objection to a dozen or more traps around the room.

If you are lucky enough to have a dedicated listening room or home theater then this concern is much less of a problem.

If you are using quality acoustic panels, then even 2 bass traps in the front corners behind the speakers should be enough to begin to hear some improvement in the room’s sound. More traps will be even better, though the key is to have a balanced approach.

One cannot have too much bass trapping, but it is very easy (and all-too-common) to have too much high-frequency absorption. One must take this into account when developing a treatment strategy.

3 A Good Room Will Have Even Bass Response

We will begin with bass response for a few reasons. First, it is the area where most rooms need the most help. Controlling the low frequency response is likely to make the most noticeable improvement in a listening room’s performance. In many rooms, there is not much clarity in the low end.

Some bass notes seem to disappear entirely, yet other notes seem to make the entire room resonate. If there are strong resonances, some rooms will have a severe ‘one note bass’ problem where no matter what key the song is in or what note is being played, the bass sounds the same.

To put these qualitative listening phenomena in a different, more quantitative way, most rooms have a jagged frequency response throughout the bass range, showing swings of as much as 40dB or more between the highest peak and the lowest null (see Fig. 1 below).


Figure 1 – This graph shows the frequency response of a typical listening room in an average home. Note the 40dB swing between the highest peak at about 60HZ and the deepest null at about 75Hz.


This uneven frequency response is caused by reflected sound waves bouncing around the room and interfering with one another. In some places and frequencies, these sound waves reinforce each other causing a peak; in other places the cancel each other out causing a null. Usually the most severe peaks and nulls are related to the room modes (frequencies related to the room’s dimensions), but there will be peaks and nulls in many places throughout the frequency spectrum.

In addition, each point in the room will have its own unique frequency response. This reason alone is why I am suspicious of electronic ‘room correction’ EQ circuits or DSP; even if the strategy works for one place in the room (this is also debatable), you are likely making things worse everywhere else in the room. Additionally, EQ or DSP will not improve low frequency ringing, which is the audible resonance causing a bass note to linger well past when it is actually finished playing out of the speakers.

Luckily there are several strategies we can employ to improve the low frequency response of a room. We will now examine each one in detail.

3.1 First Strategy: Room Setup and Layout

The first thing to look at is how the room is laid out. Again we will have more flexibility here if we are dealing with a dedicated listening room. If the room must also serve as a living room then our options may be more limited. The thing we want to avoid with layout is to make sure that neither the speakers nor the listening position are in a place with a severe null point. If this is so, then the speakers will be working harder just to reproduce frequencies that can’t easily be heard. As a result people tend to turn up the bass controls or the overall volume to compensate, which makes the amps and speakers work harder, likely increases the distortion of the system, and only amplifies the unbalanced frequency response. A better solution is to set things up so we aren’t fighting such a steep uphill battle from the get-go.

3.1.1 The ‘38% rule’

Credit goes to noted studio designer Wes Lachot for the 38% rule. In actuality, this isn’t so much a rule as a guideline, or even better as a starting point for experimentation (or measurement, for the truly dedicated). The idea behind the 38% rule is that in a simple, rectangular room, a listening position 38% from either the front or rear wall is likely to exhibit the flattest frequency response, with the least likelihood of peaks or nulls.

We can take advantage of this insightful theory by starting our room setup with the 38% rule, by placing our listening chair 38% into the room from either the front or the rear wall. Most listening rooms will have the listening chair 38% from the rear wall, in contrast to many home recording studios where the listening chair is 38% from the front wall, leaving room behind for musicians to be recorded.

Once the listening position is established, the next step is to place the speakers in an equilateral triangle with the listening position. In most cases the tweeters should be at ear level. The biggest question in this step is how large to make the triangle, or put another way how far apart the speakers should be. The answer generally is the equilateral triangle should be as large as possible, but the speakers should be as far away from the side walls and the front wall as possible. It is always a trade-off between these two factors. One aim here is to get the best possible stereo image (covered more below) but we also want to pay attention to the bass response.

3.1.2 SBIR

When a speaker is placed near a wall boundary, there is another acoustic phenomenon that can occur called Speaker Boundary Interference Response (SBIR). When bass leaves a speaker, it generally is omnidirectional, meaning it travels equally in all directions. Think of a pebble in a pond, where the wave radiates outward in a circle (though in a room it is actually a sphere, not a circle). When the speaker is close to a wall, some of that energy is reflected back toward the speaker, and the reflected bass wave can interfere with the new bass wave that is just leaving the speaker, causing the same sort of frequency anomalies seen above. Changing the distance between the speaker and the wall will affect how the SBIR behaves (or if it leaves an audible imprint at all), so some experimentation is in order.

3.1.3 Experiment BEFORE Adding Treatment

Note that it is very helpful to do all of the above experimentation BEFORE treatment is installed in the room for one simple reason. Once acoustic treatment is installed, the problems in the room become harder to hear, and the room becomes more consistent from one point to another. Without treatment, the differences in the room response are easier to identify. The idea is to get things sounding as even as possible before we install treatment, which then puts you in the best possible position to get a superior sounding room with a good treatment strategy.

3.2 Bass Traps In Corners

At the risk of oversimplification, the more bass traps you install into the more corners of the room (including wall/ceiling corners), the flatter the response of your room will get. Remember that the jagged frequency response curve of most rooms is caused by reflected bass waves interfering with one another. Adding bass traps absorbs much of this excess sound energy, thereby reducing the amount of acoustic interference that happens in the room. The more traps you add, the more of these excess reflections are absorbed, which further reduces the interference, thus flattening the frequency response of your room.

3.2.1 Why Corners?

Typically, the corners of the room (including wall/ceiling corners) will be prime locations where bass builds up in the room, which is why putting bass traps in the corners is a good strategy – there is simply more bass in those locations to be absorbed, thus increasing the efficiency of each trap. Remember that a rectangular room has 12 corners, the 4 wall/wall corners, as well as the wall/ceiling and wall/floor corners. They are all the same in terms of acoustics; the sound doesn’t know which way is up. Bass will typically build up in all corners of the room.

If your room is irregular, meaning not a simple rectangle or if you don’t have any 90-degree corners, then there are likely to be other locations in the room where bass traps might be useful. In such cases, it is often useful to do a simple listening test to find areas of bass buildup in the room. Any place in the room with a bass buildup is a good place to install a bass trap if possible.

An effective bass trapping strategy involves simply getting enough bass traps into the room – this alone is more than half the battle. Effective placement, however, can make the traps you do have perform even better. The fewer the traps in the room, the more important placement becomes. In a typical, rectangular room, I will put traps in the following locations, in order of importance:

  1. Front corners, behind the speakers
  2. Rear corners, behind the listener
  3. Front wall/ceiling and/or wall/floor corners
  4. Rear wall/ceiling and/or wall/floor corners
  5. Side wall/ceiling and/or wall/floor corners, usually starting at the front of the room closest to the speakers and working your way back from there

3.2.2 Can You Have Too Many Bass Traps?

The answer to this question depends on what is meant by ‘bass traps.’ In terms of pure low frequency absorption in a small room, the answer is no, you cannot have too many bass traps. The more traps you add, the flatter the response will get. However, it is very easy to have too much high frequency absorption in a room, resulting in a room that is too dead-sounding. Unfortunately this mistake is all-too-common in listening rooms; a typical example of this mistake is where absorptive foam is installed on all wall and ceiling surfaces, with little or no bass trapping under 200Hz. This results in a very unpleasant-sounding room that is completely dead at high frequencies, yet still exhibits much of the boominess and uneven response common to all rooms. I always feel bad when I am in a room ‘treated’ this way; a dead, boomy room sounds very unnatural and unpleasant.

Many common DIY traps, which are more accurately called broadband absorbers, are built using a 4′ or 6′ thick slab of rigid fiberglass, rockwool, or perhaps acoustic cotton insulation. While these traps are effective absorbers across the frequency spectrum, they generally absorb the same amount of treble (if not more) than bass. As a result, lining all the room corners with traps like this will often give you too much high frequency absorption, especially by the time enough traps are added to control the low end.

The solution to this problem – how to get enough bass trapping without making the room too dead at high frequencies – is to use bass traps that absorb more bass and less treble. This requires some ingenuity; at RealTraps we did a lot of R&D on specifically this problem and came up with a very elegant solution, which is one reason why our bass traps are such strong performers. We added a limp mass membrane, after testing a wide variety of materials and thicknesses, which produced exactly the results we were hoping for; now all RealTraps bass traps contain this membrane.

3.3 Rear Wall

The last place to look for treatment in a normal, rectangular room to improve the low frequency response is the rear wall, behind the listening position. This is the wall that the speakers fire toward. Most rear walls (especially in a simple rectangular room) will have 2 problems associated with it: first is the high frequency ‘bounce’ off the walls; second is that the rear wall usually has a strong bass buildup audible when you listen from the rear wall. Often, the bass buildup is correlated to a null in the middle of the room.

One way to hear this bass buildup is to do a listening test similar to the one described above. While listening to bass-heavy music or a test tone, start listening with your back all the way up against the rear wall. From here, you should hear unnaturally strong bass response. As you begin to walk forward, the bass will become less strong (and more accurate initially), up until you reach the center of the room where the bass will be unnaturally weak, it will seem to disappear. Then as you continue going forward, the bass will come back and become strong again as you get close to the front wall and the speakers. As described in 3.1.1 above, chances are the most accurate bass response will be at or near the 38% point from the front or rear wall.

If this sounds familiar to your room, then treating the rear wall can have a profound impact on the sound you hear. This is especially true for those who have trouble hearing enough bass, who are sitting near the center of their room, and who hear the strong bass buildup on the rear wall. By getting enough bass traps on the rear wall, we can flatten out the response in both positions; the bass will be less unnaturally loud in the back, and it will be less unnaturally soft in the middle. Everyone wins!

But, don’t forget we also have high frequency reflections from the rear wall to deal with. These reflections can be treated in 2 possible ways: absorption or diffusion. Both are effective, both sound slightly different, and both will have different levels of cost-effectiveness. Absorption is nearly always the most cost-effective treatment for the rear wall. This is a good place to use full broadband absorbers, that absorb equal amounts of bass and treble. In RealTraps products, this would be our ‘HF style’ bass traps, which are built without the limp mass membrane. Diffusion, as long as it is accompanied with bass trapping, is also very effective. Many prefer the sound of diffusion to absorption on the rear wall because it doesn’t remove the sound energy from the room, rather it diffuses the sound (scatters it evenly in all directions), thereby removing the ability of reflected sound to cause comb filtering. It’s quite difficult to describe the sound of diffusion with audiophile buzzwords; one really must hear it for themselves. The best description I’ve heard is that diffusion can add a nice sense of ambient ‘sheen’ to the sound. For more on Diffusion, see section 5.2 below.

With all of these strategies in place, the room will have a much more consistent low frequency response, and (arguably) the largest hurdle toward good sound in a small room will have been largely overcome.

4 A Good Room Will Have a Detailed Soundstage

With a good foundation of solid, accurate bass response behind us, we can turn our attention to the next biggest problem facing us in small room acoustics: hearing an accurate soundstage and image. Most people prefer to listen to music on quality speakers, but these days more and more people find great satisfaction listening through quality headphones (I might say earbuds here for portable mp3 players, but I have yet to hear a set of earbuds that can compete with full-size headphones for sound quality). I’ve heard many a headphone lover claim an increased sense of being enveloped by the music; the soundstage is extremely wide and detailed because the sound is ‘piped’ directly into our ears.

Most people – particularly those people who have little or no experience listening in good rooms – don’t realize that with a good listening room, you can achieve or even surpass the quality of soundstage one gets from quality headphones in a good room with good speakers. Surprised? I certainly was when this was made clear to me. But let’s take a look at what happens in a typical listening room.

The first thing the listener hears is the direct sound leaving the speaker and travelling in a straight line toward the listener’s ears. Then, a few milliseconds later depending on the size of the room and how it is arranged, the listener will begin to hear first reflections, where the sound leaves the speakers, bounces off the side walls, the ceiling, and the floor, and then hits the listener’s ears. Note that these reflections are not ‘late’ enough to be perceived as a separate echo (unless you have a very large listening room); rather they all converge and our brain tries to interpret them as the same sound.

This means that for each speaker, we hear essentially the same sound coming from 5 different places (direct from speaker, left wall reflection, right wall reflection, ceiling reflection, floor reflection). Since the reflected sounds are not ‘late’ enough in time to be perceived as a separate sound or echo, all that happens is our brain’s ability to locate where the sound is coming from gets confused. Since we have more difficulty locating sounds, the soundstage gets muddled.

So then the question becomes: how can we improve the coherence and quality of our soundstage?

4.1 Room Symmetry From Listening Position Forward

The first method has to do with how the room is laid out. We discussed this in section 3.1 in terms of avoiding bass nodes (especially null points), but we didn’t address the imaging issue. Fortunately, the advice here is simple: arrange the room to be as symmetrical as possible; this is most important from the listening position forward.

If the room is asymmetrical, then it can cause the sound to also be asymmetrical which degrades the soundstage. This asymmetry can manifest in a few ways: one is uneven bass response (if there is a much larger area to one side then the bass will be different there than in a small area). Another is that if one side wall is further from the other, then reflections to that wall will have further to travel to hit your ear, which means they will arrive later in time. This can also contribute to an unbalanced stereo image.

When deciding how to lay out a listening room, keep symmetry in mind. If possible, lay things out so that the speakers fire into the longest part of the room, with the room as symmetrical as possible, particularly from the listening position forward.

4.2 Reflection-Free Zone

Once the room is arranged with as much symmetry as possible, we still have those first reflections to deal with. In short there are two possibilities. First is to redirect them, to aim them away from your ears by angling the side walls and the ceiling. This of course requires construction and is not simple to accomplish with precision and high aesthetics. Second is to absorb the first reflections, by using acoustic panels at reflection points. This second option is nearly always preferable, unless the room is being built from the ground up with a skilled room designer who understands how to manipulate angles so they benefit, not deter, the sound.

You can find the reflection points (so you know where to put the absorbing panels) either by calculating them if you enjoy geometry, or with a simple test known as the ‘mirror trick.’ The mirror trick requires help from a friend as follows:

  • With your speakers set up, sit in your listening chair.
  • Have your friend hold a mirror on the side wall at head-level, and move the mirror around the wall until you can see the reflection of each speaker in the mirror.
  • Mark the reflection point with a piece of tape or something temporary, so you know where the panels go.
  • Repeat the test on both side walls and the ceiling, with a marker for each speaker in the system, ie, left/right for a 2-channel system, or 5 speakers for a 5.1 surround system.
  • In a larger home theater, you may wish to repeat all of the above steps so that all listeners in the theater can enjoy a reflection-free zone.

Once you have all the reflection points marked, install enough absorbent acoustic panels to cover all the reflection points. Generally this will mean 1-2 panels on each side wall, and 1-3 panels on the ceiling, depending on the size of the room, how far apart the speakers are, and the number of speakers in the system.

Don’t forget, in general you want to be judicious with high-frequency absorption as explained in section 3.2.2. You don’t want the room to be too dead! Generally once you have enough bass traps and a solid Reflection-Free Zone, you will have a tonally nicely-balanced room, and pinpoint imaging from your system. If there are any remaining trouble spots where you can hear flutter echo or something similar, you can always add additional spot treatments to deal with them.

5 A Good Room Will Have An Even Reverb

The above strategies to improve the sound of a listening room are specialized for small rooms. In the acoustics world, this means any room less than about 10000 cubic feet, though it is somewhat difficult to draw a specific line between large and small rooms. As an example, picture a very large great room with cathedral ceilings, 35’x30′ with 15′ peaked ceilings. If most of the surfaces in this room are reflective (ie, hardwood, glass, drywall, concrete) then a room this size will definitely have some reverb.

In fact, all rooms have some amount of reverb. All rooms also have low-frequency mode problems as well. Usually there is a transition point between reverb being the biggest problem (at higher frequencies) and where modes are the biggest problem (at lower frequencies). In larger rooms this transition point will be much lower than in smaller rooms.

Regardless, reverb is relatively simple to treat: the answer is always to install enough broadband absorption, more or less evenly scattered throughout the room, until the reverb time is lowered to the desired level. While this simplification is mostly true, we will now examine it more carefully.

5.1 Reduce Reverb Time, Evenly at All Frequencies

Reducing reverb time in a large, reflective room is usually a very good idea. For rooms with long reverb times – say, more than 1 second – it can be difficult to hear with much detail. In particular, home theaters in large, reflective rooms often have poor dialogue intelligibility. But part of the reason for the poor intelligibility is that reverb times can vary significantly across the frequency spectrum. So while we want to reduce reverb time, it’s important to have a similar reverb time across all frequencies.

5.1.1 A good (small) room will have 250-500 millisecond reverb times

In general, a target reverb time of 250-500 milliseconds is good for a small room, depending on the size of the room as well as its specific use. A home theater (or conference room – or any room where speech intelligibility is critical) should aim for 250ms, whereas a larger space used for music (performance or reproduction) could get by with 500ms (or even 1 second, if it is a very large space relative to a home, like a music club), provided the time is stable across the frequency spectrum. Note that the above applies to domestic-sized rooms; large concert halls known for their good orchestral sounds will often have reverb times of 1.5 to 2.5 seconds, but this doesn’t apply to the smaller rooms most of us are dealing with.

5.2 Reverb Treatment Strategies

The strategies we use to reduce reverb time in a room will be familiar in that they employ bass trapping and broadband absorption, however we will also take a look at diffusion.

5.2.1 Bass Trapping

Even in larger rooms, we still cannot neglect bass trapping. It’s true that the smaller the room, the more important bass trapping is, but even larger rooms will have some low end inconsistency that can be improved with bass trapping. The strategies are the same: put as many bass traps as possible into as many areas with bass buildup (ie, corners) as possible. The difference is, the peaks and nulls will tend to be at lower frequencies, typically well under 100Hz, in larger rooms than in smaller rooms (typically under 400Hz).

Most broadband bass traps will also have a certain amount of high-frequency absorption, which should be taken into account when predicting how much high frequency absorption to use.

5.2.2 Absorption

For high frequency absorption, a simple 2’x4′, 1′ or 2′ thick broadband absorber made from rigid fiberglass or rockwool will absorb down to about 250Hz or so provided they are built correctly. Panels like these are quite adequate to reduce reverb time; it’s just a matter of getting enough traps into the room.

Some people will want to build their own DIY traps; this can certainly be done and is common enough provided you have the craftsmanship, the plans, the tools, the workspace, and enough spare time to do the work. Many prefer to buy high quality panels from a reputable vendor that can show the absorption is effective in the desired ranges, particularly the low bass range which is the biggest challenge in building effective panels. An effective approach to reducing reverb time evenly will have a mixture of hgih frequency absorption and bass trapping, which can be tricky for the novice to navigate.

The correct number of traps needed can be calculated; an acoustician will understand how to measure the initial reverb time of the room, calculate the total surface area of the room, and then calculate how much absorption in terms of surface area will be needed to achieve the desired reduction in reverb time. However, these calculations are beyond the scope of this article; in addition it is often much simpler to take a more practical approach.

Since absorbent panels are often made from 2×4 sheets of rockwool or rigid fiberglass, it makes sense to install a fixed number of traps (say, 12 traps to begin with, or whatever quantities make sense to minimize costs), and see how much more improvement is needed. It’s possible that the first round of traps will be adequate on its own. Another possibility is that little improvement will be heard, in which case you know that many more traps are needed. Most likely is that some improvement will be clearly audible; it is up to you to decide whether or not more improvement is desirable (requiring more traps).

Again there is a practical benefit to this step-by-step approach: you stop when the sound reaches the desired reverb time; this way you won’t overdeaden the room and kill the high end. You can add to your treatment, a few traps at a time, until the desired sound is achieved.

5.2.3 Diffusion

The last type of treatment we’ll discuss is probably the sexiest – or at least the most recognizable – type of treatment: Diffusion. Diffusion is a different approach than absorption, but the end result of both styles of treatment is similar in that they eliminate reflections. Whereas absorption removes the sound energy from the room (technically it converts sound vibrations into heat), diffusion scatters the reflections, evenly in all directions. So diffusion leaves some ambience in the room, but it removes the destructive capability of the sound reflections to create comb filtering (peaks and nulls).

There is nothing magical about diffusion, but it does require some precision when building diffusors. There is a common misconception that bookshelves will add ‘diffusion’ to a room; while bookshelves will break up sound waves a bit, certainly more so than a bare wall, a bookshelf will NOT add diffusion. A QRD-style diffusor, which has wells of varying depths, are not simple random patterns, they are carefully calculated mathematical sequences. If not built precisely, it will not work as a diffusor.

Diffusion is most common on the rear wall of a listening room, for a variety of reasons. One is that the listener must be a certain distance away from the diffusor in order for it to work correctly; if a listening position is on a couch up against the rear wall then diffusion is likely to be unsatisfactory. This is also the reason I don’t generally recommend diffusion on ceilings, unless you have tall ceilings (generally at least 14-15′ tall).

Diffusion is also extremely effective placed directly behind a dipole-type speaker; it removes the ability of the rear wave to interfere with the main sound from the front of the speaker, but it does maintain the ambience that dipole-type speakers are known for.

Lastly, I never recommend using diffusors at first-reflection points (see section 4.2 above). In every test I’ve seen/heard – either subjective listening or objective measuring – absorption easily outperforms diffusion at reflection points. One can, however, get fantastic results from using diffusion throughout the perimeter of the room, with the exception of bass traps in corners and absorption at side wall reflection points. [A?] [?A] For much more detail on this strategy, see the video ‘Hearing is Believing’ on the RealTraps website: http://realtraps.com/video_hearing.htm

6 Making the World Sound Better, One Room at a Time

This article has been a comprehensive journey through the characteristics of a good sounding room. At this point I’d like to remind the reader of the 2 most basic truths about room treatment: there is no such thing as a perfect room, and every room can be improved. I encourage you to use the information in this article to improve the sound of YOUR listening room. If you need further help, I’d point you to our website at www.RealTraps.com, where you will find a huge amount of concise information (articles and videos) that explains many of these concepts in more detail. If you need one-on-one assistance, you can contact me from the RealTraps website (see the Contact page) for one-on-one consultation. Happy listening!

James Lindenschmidt is General Manager of RealTraps, where he has helped thousands of customers improve the sound of their listening and recording rooms. For more detail on many of the concepts in this article, take a look at the extensive educational material (articles and videos) available at www.RealTraps.com.

Copyright (C) 2012 James Lindenschmidt